1. Field of the Invention
The present invention relates to a technology that performs printing by forming dots on a printing medium using a printing head.
2. Description of the Related Art
Inkjet printers such as serial scan type printers and drum scan type printers perform printing using a printing head while scanning in a main scan direction, and form text and images on a printing medium by ejecting ink from multiple nozzles of a printing head.
As one dot recording method used with inkjet printers, there is a method called the xe2x80x9cinterlace method.xe2x80x9d FIG. 31(A) is an explanatory diagram that shows sub-scan feed for an interlace recording method. Printing head 10 has four nozzles placed along the sub-scanning direction. The numbers 0 through 3 noted in the circles are the nozzle numbers. Nozzle pitch k in the sub-scanning direction between nozzles is 3 dots. Here, a unit called a xe2x80x9cdotxe2x80x9d means a dot pitch in the sub-scanning direction that correlates to a printing resolution in the sub-scanning direction. In FIG. 31(A), the positions of printing head 10 noted as pass 1, pass 2, etc. indicate the sub-scanning direction position at the time of each main scan. Here, xe2x80x9cpassxe2x80x9d means one main scan. After each main scan, sub-scan feed is executed at a four dot fixed feed amount L.
FIG. 31(B) shows the ordinal numbers of the nozzles that record dots on each main scan line. As can be understand from this Figure, with the interlace recording method, even when the nozzle pitch k is 2 dots or greater, dots can be formed on all main scan lines.
However, the positions of dots formed by each nozzle sometimes shift a little bit in the sub-scanning direction due to nozzle manufacturing error. FIG. 31(B) shows a case where there is no such manufacturing error and all the dot positions are normal. Meanwhile, in a case when a dot formed by nozzle #1 is shifted vertically, for example, as shown in FIG. 31(C), a gap occurs between the main scan line formed by dots from nozzle #1 and the main scan line formed by dots made by nozzle #0. With the naked eye, this kind of gap is observed as a stripe shaped area of image quality degradation called xe2x80x9cbanding.xe2x80x9d Note that the cause of banding is not just nozzle manufacturing errors, but may include other errors such as sub-scan feed error, folds in the printing medium, etc.
To prevent this kind of banding, a recording method called an overlapping recording method is used. FIGS. 32(A) through 32(C) show the effect of the overlapping recording method. As shown in FIG. 32(A), the sub-scan feed amount L for this recording method is a fixed value of two dots. In FIGS. 32(A) through (C), the nozzle positions of the even numbered passes are shown by a diamond shape. When all the dot positions are normal, as shown in FIG. 32(B), the dot position recorded on even numbered passes are placed alternately with the dot positions recorded on odd numbered passes without any gaps. As a result, the dots on the same main scan line are formed by two different nozzles. In this way, this method of recording using multiple different nozzles to record multiple dots on the same main scan line is called an xe2x80x9coverlapping method.xe2x80x9d
With an overlapping method, as shown in FIG. 32(C), where dots formed by nozzle #1 are shifted vertically as well, we can see that the gap does not stand out as much as in FIG. 31(C). As a result, it is possible to soften the banding. To make the most of this advantage of this kind of overlapping method, it is desirable to make the number of overlaps (in other words, the number of nozzles in charge of forming dots on each main scan line) as big a value as possible.
However, the overlapping method has the problem that printing speed is slower than with non-overlapping methods. The sub-scan feed amount L in the non-overlapping method shown in FIG. 31 is four dots, while that in the overlapping method shown in FIG. 32(A) is two dots. The printing speed is approximately proportional to the sub-scan feed amount, so the printing speed of this overlapping method is approximately half that of the non-overlapping method.
With inkjet printers, there is demand for high speed printing of images at the same level of smoothness as a photograph. Achieving higher image quality can be achieved to some degree by making the dots smaller. However, when the dots are smaller, there is a tendency for the banding due to variance in nozzle characteristics to stand out more. Meanwhile, to soften this kind of banding, when the number of overlaps is increased, there is the problem that printing speed is decreased.
Accordingly, an object of the present invention is to increase image quality without excessively decreasing printing speed.
In order to attain the above and the other objects of the present invention, there is provided a method of printing by forming ink dots on a print medium. The printing method comprises the steps of providing a print head having a plurality of nozzles arrayed along a sub-scanning direction for ejecting same ink; allocating n(j) number of nozzles to a j-th main scan line in a selected area on the print medium where n(j) is an integer of two or more, the integer n(j) for some main scan lines being set at a different value from that for other main scan lines; positioning each of the n(j) number of nozzles on the j-th main scan line; and driving each of the n(j) number of nozzles, in response to given print data, to enable the nozzle to form dots intermittently at a rate of one in mxc3x97q dot positions on the j-th main scan line during one main scan, m being an integer of 1 or more, and q being an integer of 2 or more, to thereby complete dot formation on the j-th main scan line with the n(j) number of nozzles during n(j) number of main scans.
In the printing method of the present invention, the numbers of nozzles in charge of recording each main scan line are different values. For example, some main scan lines are recorded by four nozzles while other main scan lines are recorded by two nozzles. Therefore, it is possible to increase the recording speed, comparing to cases when all main scan lines are recorded using four nozzles, and the image quality can be improved comparing to cases when all main scan lines are recorded using two nozzles.
In a preferred embodiment of the invention, the method comprises the step of: executing sub-scan with a constant sub-scan feed amount of Lxc3x97P each time one main scan is completed, where P denotes a dot pitch corresponding to a printing resolution in the sub-scanning direction, and L is an integer of 1 or more, wherein a nozzle pitch of the plurality of nozzles in the sub-scanning direction is kxc3x97P where k is an integer of 3 or more, and wherein L and k satisfy equations (1) and (2):
L=fxc3x97kxc2x1gxe2x80x83xe2x80x83(1)
N=L+Rd[Rxc3x97L÷k]xe2x80x83xe2x80x83(2)
where N denotes a number of working nozzles during one main scan, N being an integer of 3 or more, f is an integer of 2 or more, g is an integer that is at least one and less than k, R is an integer that is larger than k and is not an integral multiple of k, and an operator Rd[ ] denotes a rounding operation for rounding a decimal part of a value in the brackets. This arrangement suppresses irregular colors of a low spatial frequency that can easily be recognized by the human eye, thereby improving image quality.
In another embodiment, the method comprises the step of: executing sub-scan with a variable sub-scan feed amount of Lxc3x97P each time one main scan is completed, where P denotes a dot pitch corresponding to a printing resolution in the sub-scanning direction, and L is a cyclically changing integer of 1 or more,
wherein a nozzle pitch of the plurality of nozzles in the sub-scanning direction is kxc3x97P where k is an integer of 3 or more, and
wherein L and k satisfy equations (3) and (4):
L=Lavexc2x1gxe2x80x83xe2x80x83(3)
N=Lave+Rd[Rxc3x97Lave÷k]xe2x80x83xe2x80x83(4)
where N denotes a number of working nozzles during one main scan, N being an integer of 3 or more, g is an integer that is at least one and less than k, R is an integer that is larger than k and is not an integral multiple of k, Lave denotes an average value of L in one cycle, and an operator Rd[ ] denotes a rounding operation for rounding a decimal part of a value in the brackets.
The present invention can be realized in various forms such as a method and apparatus for printing, a method and apparatus for producing print data for a printing unit, and a computer program product implementing the above scheme.
These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with the accompanying drawings.